Disk drives are widely used in computers, consumer electronics and data processing systems for storing information in digital form. The disk drive typically includes one or more storage disks and one or more head suspension assemblies. Each head suspension assembly includes a slider assembly having an air bearing surface and a read/write head that transfers information to and assembly to ride on an air bearing a distance “h” from the storage disk. The distance “h” is referred to as the “fly height” of the slider assembly and represents the position that the slider assembly occupies when the storage disk is rotating during normal operation of the disk drive.
The term “head-to-disk spacing” refers to the distance between the read/write head and the storage disk. More specifically, the head-to-disk spacing for a disk drive is equal to the sum of (i) the carbon thickness on the read/write head, (ii) the distance between the read/write head and the carbon surface of the storage disk, (iii) the carbon thickness of the storage disk, and (iv) half of the thickness of the magnetic layer on the storage disk.
Each storage disk includes one or two disk surfaces that are divided into a plurality of narrow, annular regions of different radii, commonly referred to as data tracks. The number of tracks per radial inch (TPI) on the storage disk is known as track density. Digital information is recorded on the data tracks in the form of magnetic transitions or bits using the read/write head. The number of bits per inch (BPI) along the track is known as linear density. The areal density of the storage disk is determined by multiplying the tracks per inch by the linear density. Areal density is necessarily increasing in an effort to raise the storage capacity of the disk drive, while maintaining a fixed or lower manufacturing cost of the drive.
For a given linear density, a target head-to-disk spacing is required to ensure accurate data transfer. Unfortunately, consistent head-to-disk spacing during data transfer is difficult to achieve. For example, during the process of forming the air bearing surface, the slider assembly is polished with slurry. The main material of the slider assembly is sintered alumina-titanium carbide and the read/write head consists of sputtered alumina and other soft metals. The rate of materials removal during polishing is slightly higher in the sputtered alumina and the other soft metals. This causes the surface near the read/write head to be slightly recessed. This is commonly referred to as “pole tip recession.” Additionally, alumina-titanium carbide and sputtered alumina each also have a slightly different coefficient of thermal expansion. At relatively cold temperatures, the surface near the read/write head further recesses due to this disparity in thermal expansion coefficients.
As a result thereof, for a given fly height, the amount of pole tip recession will influence the resulting head-to-disk spacing of the disk drive. For example, for a 30 Gbits/in2 areal density disk drive, the nominal pole tip recession can be approximately 4 nanometers, carbon thickness on the read/write head is about 4 nanometers, fly height can be approximately 13 nanometers, carbon thickness of the storage disk can be roughly 4 nanometers and half of the magnetic layer is about 10 nanometers. Thus, the total head-to-disk spacing in this example is approximately 35 nanometers.
Further, a large variation in fly height from slider assembly to slider assembly can cause significant issues in the manufacturing and reliability of the disk drives. If the fly height deviates positively and significantly from the target fly height, the head-to-disk spacing may become too large and can cause unreliable reading from and writing to the storage disk. Conversely, if the fly height deviates negatively and significantly from the target fly height, the head can contact the carbon surface of the storage disk.
Maintaining a relatively small and consistent head-to-disk spacing is further complicated by other factors. In particular, the read/write head includes a read head and a spaced apart write head. The write head includes an electrically conductive write element, a leading pole having a leading pole tip, a trailing pole having a trailing pole tip, and one or more electrical insulating layers that surround the write element. The space between the leading pole tip and the trailing pole tip is known as the write head gap. Further, a yoke forms an arch from the leading pole to the trailing pole. Conventional write elements often include a coil wound around the yoke. During a write operation, current is directed through the write element. The current generates a magnetic flux that propagates along the yoke to the pole. The magnetic flux generates a magnetic filed at the write head gap that is used to record digital information to the storage disk.
The electrical resistance of the write element generates heat in and around the read/write head. The extent and rate of heating depends upon the level of current sent to the write element. The temperature at the pole tips and surrounding areas of the slider assembly increases over time and eventually reaches a maximum temperature during writing. This temperature increase causes the trailing pole tip and/or other portions of the slider assembly to expand and protrude during writing in a direction generally toward the storage disk. This phenomenon is known as “thermal protrusion” or “thermal pole tip protrusion”. If the extent of thermal protrusion is as great as the fly height, the slider assembly can contact the storage disk. This contact can cause off-track writing, damage to the slider assembly, damage to the storage disk and/or a permanent loss of data. Thus, thermal protrusion limits how low the fly height can be designed for a disk drive.
Attempts to limit the extent of thermal protrusion include decreasing resistance in the write element, thereby reducing the amount of heat transferred to the trailing pole. These efforts included providing a more tightly wound write element and using thicker coil materials. Another attempt to inhibit the adverse effects of thermal pole tip protrusion includes using a single layer coil rather than the conventional two-layer coil, in order to reduce heat buildup near the coil and encourage greater heat conduction to the surrounding areas of the slider assembly. A further attempt to reduce thermal protrusion includes replacing one of the insulating layers, which normally includes a photoresist material of relatively high coefficient of thermal expansion, with material having a lower coefficient of thermal expansion, such as alumina. Still another attempt to decrease thermal protrusion includes using a thinner undercoat layer between the body section and the read/write head. Unfortunately, these attempts have met with somewhat limited success.
In light of the above, the need exists to provide a disk drive that allows for the precise adjustment and control of the head-to-disk spacing. A further need exists to provide a disk drive that allows for a relatively high fly height, thereby decreasing the likelihood of damage to the slider assembly and/or loss of data from the storage disk. Another need exists to manufacture a reliable and cost effective disk drive that provides increased accuracy during data transfer.